Question

How can I draw d -glucose in its chair conformation? Why it is the most common aldohexose in nature?

Answer 1

First convert the Fischer projection to a Haworth projection, then convert the Haworth projection to a chair form.

Explanation:

The Fischer projection of glucose is

Convert to a Haworth Projection

Step 1. Draw a basic Haworth projection with the ring oxygen at the top.

`"C-1"` is the atom to the right of the oxygen, and `"C-5"` is the atom to its left.

Step 2. Draw a `"CH"_2"OH"` on `"C-5"`.

Step 3. Draw an `"OH"` below the ring on `"C-1"` for the α form (draw it above the ring for the β form).

Step 4. Draw all the `"OH"` groups on the right side of the Fischer projection on the bottom of the ring. Those on the left go above the ring.

The `"O"` on C-5 is part of the ring.

You can omit the hydrogen atoms, so the Haworth projection for α-D-glucopyranose is

Convert Haworth to Chair

Step 1. Draw a cyclohexane chair in which the `"O"` atom replaces `"C-6"` and the bulky `"CH"_2"OH"` is in the equatorial position.

Step 2. Put all the `"OH"` groups that are "down" in the Haworth projection "down" in the chair. All the other `"OH"` groups go "up".

The chair form of α-D-glucopyranose is

The structure of β-D-glucopyranose is

Prevalence of Glucose

As you move around the β-glucose ring, you see that all the substituents are equatorial.

This is the most stable arrangement possible.

In α-glucose, only the `"OH"` at `"C-1"` is axial.

Every other aldohexose would have more axial substituents and be less stable.

Glucose is the most common hexose because it is the most stable.